Fear center is shrunken in severely autistic brains

December 5, 2006By Narelle TowieThis article courtesy of Nature News.

A hyperactive amygdala may cause its own self-destruction.

The more severe the social dysfunction of an autistic patient, the smaller the part of their brain that governs fear-response, according to a new study. The results have scientists wondering whether some of the symptoms of severe autism are due to the brain becoming so overworked that it attacks its own cells.

The amygdala a small part of the brain that governs emotional responses, such as fear is thought to be important in autism, as it helps to govern social behaviour.

To examine the relationship, Richard Davidson and his team at the University of Wisconsin in Madison, decided to try and match amygdala size to the development of autism. They took MRI scans of 28 male autistic participants ranging in age from 8 to 25 years, and calculated the volume of their amygdala.

The researchers then looked at whether the patients tended to avoid eye contact a well-known symptom of autism. Eye-tracking equipment was used to watch how the participants reacted when looking at images of emotional faces: autistic children tend to avoid the eyes in such pictures, and are slower at distinguishing facial expressions.

They then compared the extent of eye-contact avoidance and the particpant's age with the size of their amygdala. The results showed that the most severely affected, older subjects had the smallest fear centres in their brain.

"Older kids with smaller amygdala are the ones that tend to avert their gaze more," says Davidson.

In general, young austistic children have relatively large amygdalas. But older, teenage autistics have relatively small amygdalas. The more severe their symptoms, the new work shows, the smaller this part of the brain.

The results are consistent with the theory that a hyper-excitable amygdala in young autistic children may result in cell death and the shrinking of this part of the brain. Davidson plans to study the cells in more detail to test this theory.

It wouldn't be the first time it has been suggested that the brain may attack itself in response to a mental condition. Previous studies have shown that the amygdala is smaller in cases of severe recurrent depression, perhaps as a result of early hyperactivity.